This invention relates to a monitoring device for double threads in warp tying machines having a tying device, grippers for the groups of warp threads to be tied together, and a device for separating the outermost warp thread to move it out of the plane of the warp.
It is known to ensure the correct sequence of individual threads within the warp by inserting a so-called lease into the warp, in most cases with the aid of two leasing cords. In a device of this kind, as described in Swiss Pat. No. 619,011, the separating devices separate the outermost warp thread from the layer of warp threads by changing their position in relation to the layer of the warp threads together with the leasing cords after separation of a thread so that all threads following the next outermost warp thread are displaced by the latter. In this case, therefore, the separation of double threads is prevented by the cooperation of the leasing cords with the separating device.
Additional safety against the separation of double threads is achieved according to Swiss Pat. No. 348,937 by designing the separating device, e.g. a notched needle, in such a manner that it can only separate one thread.
If, however, a lease is used, then the prevention of separation of double threads depends solely on the correct choice of separating needle in accordance with the given yarn diameter. Even then, however, two or in exceptional cases even more than two warp threads are liable to be separated, especially if the warp threads have been sized. This has the undesirable result that the dying device produces a so-called three-legged knot. Two different types of problems then arise, depending on the position of the double thread:
If the double thread lies on the side of the old warp, i.e., if two warp threads of the old warp are tied to a warp thread of the new warp, then thread breakage occurs at the latest at the level of the first row of drop wires since the two threads of the double thread are drawn into different drop wires. This thread breakage must be repaired by the manual insertion of a piece of thread. In addtion, a missing warp thread must be supplied to the new warp from an edge of the group of threads for the second thread of the double thread.
If the double thread lies on the side of the new warp, i.e. if a warp thread of the old warp is tied to two warp threads of the new warp, then these two new warp threads together with one old warp thread are drawn through the associated drop wire, through a heald and through a tube of the reed. The fault is then not detected until the starting place is reached and one of the two new warp threads of the double thread must be cut off, pulled backwards out of the reed, heald and drop wire and deflected and removed at one side of the beam.
Apart from the fact that elimination of both these types of faults requires an undesirable amount of manual operation, the double threads are liable to impair the quality of the woven fabric. For example, if the number of warp threads which have had to be deflected and removed from the side exceeds a certain value, such as four, differences in tension are then liable to occur in the new warp resulting in a poor quality of weave.
It is therefore necessary to prevent the formation of three-legged knots. In other words, double threads must always be detected in good time before the tying process is performed.
The problem of detecting double threads is also found in other textile fields, for example in winding. German Auslegeschrift No. 1,560,548 discloses a winding machine having a monitoring device for the tying of double threads. The device comprises a measuring instrument for measuring the thread dimensions in the path of the thread downstream of the tying device. Measuring of the thread dimension may be carried out, for example, by optical means.
Optical measurement of the thread dimension always entails a certain risk of error due to dust formed in yarn processing. Moreover, it is not ideal to place the measuring instrument downstream of the tying device as this cannot prevent the formation of a three-legged knot.
The invention serves to provide a monitoring device for double threads which will not only effectively detect double threads and hence prevent the formation of three-legged knots but is also substantially immune to the presence of dirt and dust.